1. Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the uniform initiation of migration by enhancing the penetration of melts into the material at the surface where fine liquid wires are being migrated.
2. Description of the Prior Art
W. G. Pfann in U.S. Pat. Nos. 2,813,048 and 2,739,088 describes methods for practicing the movement of melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting. However, molten line and droplet in stability resulted in the breakup of the migrating lines and droplets and consequently acceptable semiconductor devices were not always obtainable.
Recently, Thomas R. Anthony and Harvey E. Cline discovered that preferred planar orientations of the surfaces of the body of semiconductor material, migration axis and line orientation axes relationships were also a necessity to migrate liquid metal wires and/or droplets through the solid body (See U.S. Pat. Nos. 3,899,362 and 3,904,442, for example.) These improvements in TGZM resulted in commercialization of the process. However, as the width of the lines being migrated becomes smaller, the penetration of fine liquid lines of less than 2 mils in width, and preferably 1 mil in width, and small liquid droplets, less than 6 mils in diameter, from the surface of a wafer or body of semiconductor material has been difficult to achieve repeatedly on a commercial basis by a thermal gradient alone. Although a thermal gradient is strong enough to cause migration of the small liquid zones once they are formed in the bulk of semiconductor material, the thermal gradient force is not powerful enough to overcome the surface tension forces holding fine liquid zones, or wires, on the surface of a body, or wafer. Despite further improvements to the TGZM processing techniques, including alloying the deposited metal to the surface (U.S. Pat. No. 3,897,277) and sintering of the same (U.S. Pat. No. 4,006,040), the problem still persists as one attempts to migrate fine wires on a commercial basis. As a result, TGZM to date has been limited to line dimensions typical of isolation grids in solid state power devices and has not had any commercial impact on integrated-circuit type devices which require much finer doped region geometries.
Therefore, it is an object of this invention to provide a new and improved method to migrate fine molten lines or wires of metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) processing which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved method to migrate fine molten wires of metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) processing which utilizes an offaxis thermal gradient.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.